lecture 5. rifting, continental break-up, transform faults how to break continent? continental...
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Lecture 5. Rifting, Lecture 5. Rifting, Continental break-up, Continental break-up,
Transform faultsTransform faults How to break continent?
Continental transform faults
Dead Sea transform
Dead Sea pull-apart basin
Continental break-up
Continental break-up
Continental break-up
Continental break-up
Continental break-up
How to break continent?
Buck (2006)
Cold lithosphere is
too strong
Buck (2006)
Effect of magma-filled dikes
Buck (2006)
Effect of magma-filled dikes
Not enough if lithosphere is thicker
than 75 km.
Effect of magma-filled dikes and lithospheric erosion
It works if lithosphere is first thinned to
about 75 km
Effect of oblique rifting
Effect of oblique rifting
(more during visit to Section from Sasha Brune)
Lithospheric thinning above Lithospheric thinning above mantle plume mantle plume
Sobolev et al. 2011
To break a continent are required:
(1) extensional deviatoric stresses (internal, from ridge push or slubduction zones roll-back) and (2) lithospheric weakening
Large Igneous Provinces are optimal for lithospheric weakening, as they may both thin lithosphere and generate magma-filled dikes.
Intensive strike-slip deformation is also helpful
Conclusion
Continental transform faults
(case Dead Sea Transform)
Continental Transform Faults
San Andreas Fault
Dead Sea Fault
Regional settingWith the surface heat flow 50-60 mW/m2, the DST is the coldest continental transform boundary
Regional setting Very low heat flow 50 mW/m2
Lithospheric thickness and magmatism Magmatism at 30-0 Ma
Lithosphere-asthenosphere boundary (LAB) from seismic data
Mohsen et al., Geophys. J. Int. 2006
LAB depth 70-80 km
Chang et al, GRL, 2011Chang and Van der Lee, EPSL, 2011
Inconsistent with the heat flow of 50-60 mW/m2
Hansen et al., EPSL 2007
Present day lithospheric thickness
Mohsen et al., Geophys. J. Int. 2006
LAB depth 70-80 km
Inconsistent with the heat flow of 60 mW/m2 or less!
Lithosphere around DST was thinned in the past and related high heat flow had not enough time to reach the surface
Conclusion
Model setup
Flat Earth approximation
Lagrangian 2.5D FE
Eulerian FD
x1
x2
x3
Simplified 3D concept.Modeling technique LAPEX 3D combining FE and FD(Petrunin and Sobolev, Geology, 2006, PEPI, 2008)
Initial lithospheric structure:
Moho map LAB map Heat flow
Modeling results: role of the thermal erosion of the lithosphere
Model setup
Flat Earth approximation
Modeling results: role of the thermal erosion of the lithosphere
Initi
al s
tage
:
5-7
km n
et s
lip
erosion at 10 Myr
no erosion
Applied force is 1.6e13N/m
Modeling results: role of the thermal erosion of the lithosphere
Ther
mal
ero
sion
of
the
litho
sphe
re
Faul
t ini
tiatio
n, 1
5-20
km
net
slip
erosion at 10 Myr
no erosion
Modeling results: role of the thermal erosion of the lithosphere
Fault development,
35-40 km net slip
erosion at 10 Myr
no erosion
Modeling results: role of the thermal erosion of the lithosphere
Present day,
107 km net slip
erosion at 10 Myr
no erosion
Possible scenario
Chang and Van der Lee, EPSL, 2011
Plumes at 25-35 Ma
Lithospheric erosion 20-30 Ma
Localization of the DST 15-17 Ma
Lithospheric erosion has triggered the DST
San Andreas Fault System
San Andreas Fault System
USGS Professional Paper 1515
Big BendSalinian Block
Bay Area Block
Hayward & Calaveras Faults
Tectonic Evolution of SAFS
N
S
Time
(animation by T. Atwater)
Questions addressed
Why the locus of deformation in SAFS migrates landwards with time?
How differently would evolve SAFS with “strong” and “weak” major faults?
Why the locus of deformation in SAFS migrates landwards with time?
Extended 2D Model Setup (South view)
T=1300°C , Vy=0, dV x/dy=dVz/dy=0
dT
/dx=
0,
Vx=
Vz=
0,
dV
y/d
x=0
dT
/dx=
0,
Vx=
0,
d
Vy/
dx=
0V
z= -
35
mm
/yr Pacific p late North America plate
Y
X
Z
G reat Va lleyC oast R anges
W E
T=0°C , free s lip , no load
0 2 5 5 0 7 5 10 0 1 25
80
60
40
0P
acifi
c pl
ate
slab w indow
fe ls ic upper crus t
1 05 km
km
T, °C02
pe ridotite
m afic low er c rus t
120010008006004002000
Y
• Eastward migration of maximum temperature and minimum lithospheric strength (Furlong, 1984,1993, Furlong et al., 1989)
T°C
0 2 5 50 7 5 10 0 1 25
80
60
40
20
0
12 0010 0080 060 040 020 00
5 M yr
upper crust
lower crust
T°C
0 2 5 50 7 5 10 0 1 25
8 0
6 0
4 0
2 0
0
12 0010 0080 060 040 020 00
20 M yr
Distance, km
Dep
th,
km
0 25 5 0 7 5 10 0 12 5
80
60
40
0
Pa
cific
pla
te
slab w indow
fe ls ic upper crust
1 05
km
02
perido tite
m afic low er crust
80
60
40
20
0
0 M yr
De
pth
, km
Dep
th,
kmW E 2.0 M yr
5 0
4 0
3 0
2 0
1 0
0
0 5 0 1 0 0 1 5 0
De
pth,
km
V,
mm
/yr
z
4 0
0
-40
G reat ValleyC oast RangesW E
5 0
4 0
3 0
2 0
1 0
0
0 5 0 1 0 0 1 5 0
9.9M yr40
0
-40SAF Eastern Fau lts
5 0
4 0
3 0
2 0
1 0
0
0 5 0 1 0 0 1 5 0
D istance, km
SAF
20.0 Myr40
0
-40
SAF
log ( ,s )-1.
1 2 .5 1 4 .4 1 6 .3 1 8 .2
Model Setup ( view from the North )
North American Plate
Slab window
Mendocino Triple Junction
140-250 km
80 k
m
Gorda Plate
N3.5 cm/yr
4 cm/yr 3.5 cm/yr
Physical background
Balance equations
Momentum: 0
Energy:
iji
j
i
i
g zx
qDUr
Dt x
1ˆ Elastic strain:
21
Viscous strain: 2
Plastic strain:
el vs plij ij ij ij
elij ij
vsij ij
eff
plij
ij
G
Q
Deformation mechanisms
Popov and Sobolev (2008)
Mohr-Coulomb
Numerical background
Discretization by Finite Element
Method
Fast implicit time stepping
+ Newton-Raphson solver
Remapping of entire fields
by Particle-In-
Cell technique
Arbitrary Lagrangian-Eulerian kinematical
formulation
11
k k k ku u K r
r Residual Vector
rK Tangent Matrixu
Popov and Sobolev ( 2008)
“Strong faults” model: the friction coefficient decreases only slightly (from 0.6 to 0.3) with increasing plastic strain
“Strong” and “weak” faults models
2-D Therm om echanical M odelling
Explicit finite element algorithm
Basic calculational cycle:
m ·d /dt = V F
F
- solution of full dynamic equation of motion
- calculations in Lagrangian coordinates
- remeshing when grid is too distorted
- no problems with highly non-linear rheology
General model setup
Complex visco-elasto-plastic rheology
T=0, =0xz = zz
T or , 0 xz zz = , - Archim.force T/ z = const
T/ x=00xz = T/ x=0
0xz =
VxVx
Governing equations:
tLAxT
xtTC
plasticityCoulombMohrordtd
G
xvKt
p
gxxp
tv
ijijiip
ijijij
ii
ij
ij
iiinert
)(
21
21“Weak faults” model: the friction coefficient
decreases drastically (from 0.6 to 0.07) with increasing plastic strain
3D Model Setup (12-15 MA)
Great Valley Block
Monterrey Microplate
Gre
at V
alle
y B
lock
Monterrey Microplate
North American
Plate
Salinian Block
Salinian Block
NW
SE
250km
1100 k
mGorda Plate
Pac
ific
Pla
te
Qualitative comparison of basic fault features
Salinian
CalaverasHayward
Bay AreaBig Bend
Salinian
CalaverasHayward
Bay Area
Big Bend
San Andreas
San Andreas
Fault Map
Modeling Results
(present day)
Salinian
Calaveras
Hayward
Bay Area Big
Bend
San Andreas
Fault Map
Bay Area
Modeled surface heat flow
“Weak faults” versus “strong faults” model
Weak-faults model
Salinian
CalaverasHayward
Bay AreaBig Bend
Salinian
CalaverasHayward
Bay Area
Big Bend
San Andreas
San Andreas
Fault Map
Modeling Results
(present day)
Salinian
CalaverasHayward
Bay Area
Big Bend
San Andreas
Fault Map
Modeling Results
(present day)Strong fault
model
Major faults in SAFS must be weak!
Strong-faults model
Present day structure and landward motion of SAFS is controlled by kinematic boundary conditions and lithospheric heterogeneity, including captured Monterray microplate
Conclusions for SAFS
Major faults at SAFS must be “week”