lecture 5. rifting, continental break-up, transform faults how to break continent? continental...

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”