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Stretching:
• Stretching causes three types of isostatic
readjustment:
• 1) Isostatic readjustment to changes in the lithosphere thickness (initial subsidence);
• 2) a thermal perturbation, which produces (by ) h
l
b
h h
d
(b
removing the excess heat out of the lithosphere) a long term thermal subsidence;
long term thermal subsidence;
• 3) deviation of basin depth from a local isostasy
will cause the lithosphere to be in an up‐ or down
will cause the lithosphere to be in an up
or down‐
ward state of flexure.
Depth of the Moho
•
Pure shear: this model assumes that deformation on the scale of the lithosphere is homogeneous
the lithosphere is homogeneous. The crust deforms more or less brittle, whereas the mantle deforms in a ductile mode. The crustal thickness distribution over
crustal thickness distribution over the rift zone gives a measure for the stretching factor 
•
Simple shear: this model assumes that thinning of the lithosphere occurs by shearing along a master fault that cuts the entire lithosphere. In this way an asymmetric rift develops. Characteristics of this mode of deformation are:
1. Flank uplift takes place only in the hanging wall of the master fault.
2 Thermal subsidence is largest
2. Thermal subsidence is largest on the location of the mantle upwelling.
•
•
Following McKenzie (1978): Uniform stretching model
Uniform stretching model • Crustal
Crustal stretching causes subsidence, whereas stretching causes subsidence whereas
thinning of the upper mantle causes uplift so in order to calculate the isostatic
in order to calculate the isostatic
compensation we need to make the following assumptions:
‐T distribution is linear
‐stretching is instantaneous
hi i i
‐deformation is homogeneous
Initial subsidence
• Crustal
Crustal stretching causes subsidence wherears
stretching causes subsidence wherears
thinning of the mantle lithosphere produces uplift Assumptions:
uplift. Assumptions:
1) T distribution of the lithosphere is linear
2) S
2) Stretching is instantaneous
hi i i
3) Deformation is homogeneous
4) Initial heat production is zero.
Density effects
1) Thinned crustal lithosphere is replaced by 1)
Thinned crustal lithosphere is replaced by
denser mantle lithosphere
2) Thinned mantle lithosphere is replaced by
2) Thinned mantle lithosphere is replaced by less dense mantle astenosphere
3) C
3) Crust is replaced by water (or sediments). The i
l db
(
di
) Th
height of the water column is the amount of subsidence.
b id
Following McKenzie (
• In time 2 the li h h
lithosphere will ill
subside because of isostatic
compensation compensation
(you replace denser mantle lid with less dense asthenosphere.
• In time 3 the lithosphere cools p
and thickens as warm asthenosphere
converts in cool t i
l
lithosphere (thermal subsidence)
In isostatic equilibrium (i.e. local compensation)
• The
The summed masses of any column of the earth above summed masses of any column of the earth above
an equipotential surface in the astenosphere must equal that of all other colums:
‐if two locations are compared the sum for all the layers of the individual mass differences must be zero; i.e.
An equipotential surface in the astenosphere exists because it has a low enough viscosity to flow i
 Δ( h)  0
i
0
i
 Δh  Δ elevation
i
0
(differences in thicknesses of layers in a column produce differences in elevation)
0‐i: levels of different densities; : difference between colums; : density; h: thickness of each level
• B
By balancing the forces acting on the b l i th f
ti
th
lithospheric columns before and after stretching for values of:
t t hi f
l
f
‐crust: 2.8 g/cm3
‐lithospheric mantle: 3.3 g/cm3
you will see that a thickness of 15km corresponds to 0 subsidence; i.e. >15km positive subsidence and <15km negative subsidence (uplift)
Based on b and the dip of master fault
Based on b and the dip of master fault
• Discrete
Discrete continental rift (e.g. Rhine Graben), continental rift (e g Rhine Graben)
normal thickness crust, extend slowly (<1mm/yr) over long periods of time (1—30
(<1mm/yr) over long periods of time (1
30 Myr) with low total extensional strain (<10km) Master fault angles (45‐70
(<10km). Master fault angles (45
70o)
• Supradetachment basins, occurred on thickened crust extend quickly (<20 mm/yr)
thickened crust, extend quickly (<20 mm/yr) over short periods of time (5‐12 Myr) with higheer amount of total strain (10‐80km).
amount of total strain (10 80km)
Different  factors
Finite strength of the lithosphere d
during rifting: Depth of Necking
f
h f
k
• Necking=large scale thinning of the lithosphere caused by mechanical extension.
• Zneck is defined as the depth of the lithosphere that remains horizontal during thinning if the effects of
remains horizontal during thinning if the effects of sediment and water loading are removed (in McKenzie model is 0km).
• (i.e the level of zero vertical motion for lithosphere stretching in the absence of gravity or isostasy. t t hi i th b
f
it
i t
• For level of Zneck deeper than the level of compensation (CD), an upward load acts on the lithosphere and an upward state of flexure results with flexurally supported rift
flexure results with flexurally supported rift shoulders
• For level of Zneck shallower than the level of compensation (CD), a p
downward load acts on the lithosphere and a downward state of flexure results. Growth of Normal Faults
Growth of Normal Faults
Isolated fault growth by radial tip propagation
Plan view
Plan view
Fw uplift generally < Hw subsidence
Area affected by footwall uplift
Area affected by hangingwall subsidence
Isolated fault growth by radial tip propagation: D L scaling
propagation: D‐L scaling
•Fault development with characteristic bell shaped
characteristic bell‐shaped D‐L (displacement‐length) profiles
•Idealised fault growth pattern
After Cartwright, Trudgill, Mansfield, 1995
Wasatch Fault, Utah
Wasatch Fault, Utah
D
L
Wasatch Fault, Utah
D
L
But, it is unrealistic to consider a single fault in isolation........
Wasatch Fault array
Wasatch Fault array
Wasatch Fault array
Wasatch Fault array
How do fault segments interact?
Segment linkage model 1. Isolated radial propagation
Segment linkage model Relay ramp
2. Overlapping interaction
Segment linkage model Sediment supply
Sediment supply
Relay ramps can be important in sediment supply
sediment supply
Segment linkage model Breached relay ramp
Breached relay ramp
Abandoned splay
3. Through‐going linkage
Interactions = Skewed displacement profiles fil
Bell shaped D L profile
Bell‐shaped D‐L profile
1. Isolated propagation
Asymmetric D L profiles skew toward each other
Asymmetric D‐L profiles –
skew toward each other
2. Interaction
D‐L profile aiming toward idealised bell shape
3. Through‐going
linkage
Tectonics builds relief and generates a b i
basin
Extension
‚half graben‘
Footwall
Hangingwall
Tectonic
subsidence
Normal Fault Growth
•
Many studies (field, sub‐surface, analogue, numerical) suggest that extensional systems are complex interplay of linkage and propagation
North Sea Mcleod et al., 2000
Volcanic Tablelands Volcanic
Tablelands
Dawers et al., 1993
Numerical modelling Cowie et al., 2000
Integrated fault growth models (
(Cowie et al. 2000)
)
Lots of small faults
A few large faults
Distinctive horst and graben structure of the canyonlands
Sedimentation and Normal Faults
Sedimentation in rift basins
• Rift basin fill normally consists of continental deposits:
–
–
–
–
–
Fluvial;
L
Lacutrian;
i
Alluvial fans;
Deltas;
Turbidites.
• Evaporites may form in rift valleys:
may form in rift valleys:
‐episodic invasion of the sea;
‐closed basin.
l db i
• Volcanic rocks may be present.
Passive margins
Passive margins
• Strongly
Strongly stretched crust over a distance of 50
stretched crust over a distance of 50‐
500 km;
• seaward thickening sedimentary prism;
seaward thickening sedimentary prism;
• they are normally characterized by shallow marine deposits (shelf deposits either clastic i d
i ( h lf d
i ih l i
or carbonate).
Erosion and subsequent
condensed section deposited
onto footwall high
Asymmetric syn-rift fill
POST-RIFT
SYN RIFT
SYN-RIFT
FOOTWALL
FOOTWALL
HANGINGWALL
HANGINGWALL
PRE-RIFT
SERIES OF DOMINO - STYLE HALF GRABENS
• Hangingwall basins
Sediment dispersal system is focused in the overlap (relay zone)
A common feature of hangingwall basins is
the vertical stacking of delta progradation
units whose architecture depends on:
1) Tectonic subsidence;
2) Glacio-eustatic fluctuation;
3) Sediment supply fluctuation to the delta
front.
Schematic delta architecture
Schematic delta architecture
Role of fault interactions in controlling synrift p
p
sediment dispersal patterns
‐Fault
Fault linkage lead to drainage linkage lead to drainage
reorganization.
‐Changes in displacement control accommodation space.
After Gupta et al. (1999)
Stratigraphy of the Gulf of Suez half graben
Delta complex
Eastern basin margin
Basin deepening
Onset of extension
Onset of extension
Individuate:
•main faults;
i f lt
•pre‐rift and syn‐rift strata; •and syn‐rift unconformity.
ENE
WSW
Bedrocks
50km
Stratigraphy of the Gulf of Suez half graben
Eastern basin margin
Basin deepening
Onset of extension
Onset of extension
Individual delta wedges (Gilbert delta type) are 15‐30m thick and comprise:
•forest unit, steeply dipping, medium‐ to thick‐ bedded conglomerates. These facies show rapid downtransport facies transition inot poximal pro‐delta bottomset sandstone turbidite and conglomerate debris flows.
The Gulf of Suez example comprises:
•At least 10 vertically stacked Gilbert delta;
•At least 10 vertically stacked Gilbert delta;
•The vertically stacked Gilbert deltas represent repeated episodes of delta progradation
d
d
fd l
d
punctuated by db
abrupt transgression of the delta top and drowning of the prograded
p g
delta. They are the result of basin y
accommodation due to fault growth.