Download Benchmark (IMEDL 2004)

BENCHMARK (IMEDL 2004)
L. L. Lavier, G. Manatschal, O. Müntener.
Dynamic modeling of rifting


Physical approach (parameter space
analysis).
Use of the physical parameterization to
interrogate the geology (Benchmark
exercise?).

What’s working and what’s not.

What is needed to improve the geological
approach?
Some background…
Rheologies

Elastic
Visco-elastic Maxwell
(Non-Linear Creep Laws)

Elasto-Plastic
(Mohr-Coulomb, the material
has both cohesional and
frictional strength)

FLAC, Fast Lagrangian analysis of
continua (Podlatchikov, Poliakov).

Self-consistent dynamic model of the lithosphere
(state of stress, strain, strain rate, viscosity,
temperature).

Spontaneous localization of shear zones.

Takes into account the weakening phenomena on
faults (non-associative plasticity).

Can model a brittle (elasto-plastic) media coupled
to a ductile (non-linear visco-elastic).

More realistic rendering of geological states.
Two Controlling Processes:
THIN BRITTLE LAYER.
-The elastic-plastic bending
of the brittle layer as the
fault offset (Force % to the
thickness square).
-The weakening on the fault
(Force % to the thickness).
Two Controlling Parameters:
-The thickness of the brittle
Layer, H.
-The rate and the amount of
weakening on the fault.
THICK BRITTLE LAYER.
Lithospheric deformation


SINGLE FAULT
LOCALIZING
(fault weakens).


MULTIPLE FAULTS
DELOCALIZING
(fault strengthens).
Weakening on faults by
strength loss (cohesion
and/or friction loss).
Elastic and Plastic bending
of a brittle layer to buildup
topography.
Weakening by thermal
necking.
Viscous strengthening in
the ductile layer by cooling
or higher strain rates.
Weakening by magmatism
(dyking).
DETACHMENT FAULTING AND MANTLE EXHUMATION:
MID-ATLANTIC RIDGE (with Roger Buck).
FORMATION OF A SINGLE NORMAL FAULT BY
EXTENDING AN ELASTIC PLASTIC LAYER
Dynamic Modeling Approach

Use of the physical parameterization with constraints
from geological reconstruction.

What are the initial and boundary conditions?

What processes control the evolution of deformation in
the plate and at the plate boundary?


What forces are needed to drive deformation?
Comparison of the model to data.
In this magma poor environment,
what is the role of detachment
faulting?




Constraints from reconstructions (Alps and Iberia
abyssal plain).
What are the initial and boundary conditions?
What processes control the evolution of
deformation at the ocean-continent transition?
What forces are needed to drive deformation and
mantle exhumation at continent-ocean transition?
2D example…
UPWARD VS. DOWNWARD CONCAVE FAULTING
Initial conditions
1cm/yr
Model space



We vary the initial temperature of the
mantle.
Temperature dependent melt fraction is
modeled. Increasing melt fraction
decreases both the density and viscosity
of the mantle.
The parameters controlling fault formation
in the brittle layer are also varied.
EVOLUTION OF THE DEFORMATION
asymmetric extension
Thinning of the continental crust (≤10 km) and mantle
exhumation over 4.3 myr
The force needed to stretch the
lithosphere is large (>2e13 Nm-1)

The force is dominated by bending of the strong brittle
parts of the lithosphere (lithospheric mantle)

The asymmetry is possible when the detachment fault
becomes very weak.

The serpentinized mantle (weak plastic) controls the strain
history (listric faulting vs. concave downward faulting).

The topography developed is unrealistic.

The depth extent of the detachment is too large (i.e. the
temperature is too low or the mantle is much weaker).
EVOLUTION OF THE DEFORMATION
RIFTING: KINEMATIC CONSTRAINTS




How does the crust thins down to 10km
before the exhumation of the mantle?
What is the effect of preexisting weakness
(suture zone, orogeny) in the lithosphere?
What is the role of the lower crust if it is
partly composed of gabbros?
What is the role of the role of the strength
of the mantle (wet or dry)?
Initial conditions
The mantle is
serpentinized when it is
uplifted close to the
surface (depth < 10 km
for a temperature < 500
°C) and along the shear
zones (high plastic
strain; friction coefficient
 = 0.2).
Model space

We vary the initial strength of the mantle.

We vary the strength of the lower crust.

The parameters controlling fault formation
in the brittle layer are also varied.
ASYMMETRIC
Gabbro-Rich Lower Crust
Weak Mantle (wet)
- The crust is thinned down to 25 km.
- The mantle is exhumed along a rolling hinge.
- Serpentinized mantle occurs when the mantle is exhumed.
EVOLUTION OF THE DEFORMATION
Total force needed for stretching
Asymmetric
Conclusions




The strong gabbro rich lower crust keeps the
deformation distributed and the topography
small. It also allows for the initial uniform
stretching of the lithosphere.
Serpentinization and weakening both control the
final asymmetry.
A weak (wet) mantle reduces the thickness of the
brittle lithosphere and the force needed to stretch
it.
A strong mantle with a preexisting weakness
leads to the formation of a symmetric rift.
What doesn’t work?



The gabbroic bodies may be distributed
heterogeneities and very strong since they
are likely to be generated in the Permian
(Ivrea body).
The initial thickness of the crust should be
maximum 30 km.
The resolution of the models is too low.
Ivrea body
Ductile shear zones in the lower crust
Snoke et al., 1999
Do we have to take into account
the Permian Collapse?
NewFoundland and melt
generation?
Conclusions




We want vary the strength and extent of the
gabbroic bodies in the lower crust
(heterogeneities).
Using this approach is walking on a thin line.
Heterogeneities can impose the physical behavior
and therefore not teach anything about the
physics.
It must remain a study of the physical processes.
If not the method becomes a “mélange
approach”.
Iterative process between the geologists and the
modelers. Between the models and the data.
CURRENT WORK


Modeled thermal history and PTt paths can be
compare to data.
Increase the model resolution and improve
modeling technique (mesh refinement + implicit
solvers) (with Wolfgang Bangerth at UT).

Model melt migration and compaction (with Chad
Hall at Caltech)

3D dynamic model of lithospheric deformation
(with Mike Gurnis at Caltech).
I have a dream…
Lower crustal flow and faulting
PTt and thermal history
Melt percolation and compaction
3D dynamic modeling
Was this phase of extension similar
to the Basin and Range?
1. Lower crustal front propagation
16 Myr.
Particle Paths
STRONG THRUST FAULT
OR LARGE THICKENING.
WEAK THRUST FAULT
OR LESS THICKENING.
MODEL RESULTS NEAR THRUST FOOTWALL
MODELS OF MELT FLOW
With C. Hall at Caltech
WHAT DOES IT TAKE TO DO
THAT IN 3D?

GEOFRAMEWORK TO COUPLE CODES TOGETHER
(MANTLE CONVECTION AND LITHOPHERIC
DEFORMATION)
1- Pythia (Python bindings)
2- StGermain (VPAC).

FLAC 3D (SNAC).

New numerical techniques.
SIMULATION OF MULTI-SCALE DEFORMATION
IN GEOPHYSICS (with Mike Gurnis at Caltech).
GEOFRAMEWORK PROJECT.
Numerical Method (Flac3D).




Explicit Finite Difference Scheme.
FLAC takes advantage of the fact that finite
difference equations can be derived for elements
of any shape (Wilkins, 1964) like finite elements.
No costly iteration process needed even for
nonlinear constitutive laws.
Need to have some a priori idea of the system
behavior to make sure the solution is stable.
3D localization first tests
Coupling between lithosphere and
astenosphere.
Red Sea test case.
Mesh refinement techniques at UT
with Wolfgang Bangerth.
Conclusions





THIS TYPE OF STUDIES IS DIRECTED AT UNDERSTANDING
AND QUANTIFYING THE FACTORS, PHYSICAL PROCESSES
AND FORCES DRIVING PLATE TECTONICS.
THEY PROVIDE A DYNAMIC IMAGE OF THE EVOLUTION OF
THE DEFORMATION AT PLATE BOUNDARIES.
THEY CAN ALSO PROVIDE CONSTRAINTS ON SUCH
PROBLEMS AS THE THERMAL EVOLUTION OF BASINS AND
SEDIMENT SOURCE AND SINK.
GEOFRAMEWORK IS NOW A SCIENCE APPLICATION IN THE
TERAGRID FRAMEWORK WITH TACC (Texas Advanced
Computer Center).
POSSIBILITY OF INTEGRATING GEODYNAMIC MODELING
WITH KINEMATIC MODELS FOR STUDIES IN THE SOUTH
ATLANTIC.
Approach
Reconstruct the stratigraphy, morphology and
paleo-water depth through time (backstripping +
palinspastic reconstruction).
2D and 3D numerical Modeling of the Tectonic
and Thermal History of the Margins.
Combine Reconstructions and Tectonic and
Thermal constraints from numerical models to
determine the maturation and migration of
hydrocarbons.
West African Margin
Kinematical approach


Successful at constraining the
tectonic and thermal history during
the post-rift phase of margins’
formation.
The assumptions for the syn-rift
history are too simplistic.
Opening of the South Atlantic

Need for seismic refraction data (Harm Van Avendonk).
Collaboration with GXT.

Constraints from the geology and plate reconstructions.


Great variability in styles of rifting. 2D numerical models of
extension along the conjugate margins. Focus on the
thermal state and the possible heterogeneities in rheology
with depth.
3D model of the opening with initial conditions taking into
account the great geological variability along the panAfrican belt.
What is the future of geodynamic
modeling?



2D models of the conjugate margins.
Study similar to the Iberian-New Founland
conjugate margins.
3D models of the opening of the SouthAtlantic (including geological constraints
from plate reconstructions and rheological
and crustal heterogenities).
Parameter space analysis.