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					   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                 MULTIPLE FAULTS

    LOCALIZING                  DELOCALIZING
    (fault weakens).              (fault strengthens).

    Weakening on faults by        Elastic and Plastic bending
    strength loss (cohesion       of a brittle layer to buildup
    and/or friction loss).        topography.

    Weakening by thermal          Viscous strengthening in
    necking.                      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




                                  3D dynamic modeling
Melt percolation and compaction
Was this phase of extension similar
    to the Basin and Range?
1. Lower crustal front propagation
                                 16 Myr.
             Particle Paths




STRONG THRUST FAULT    WEAK THRUST FAULT
OR LARGE THICKENING.   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 pan-
    African 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 South-
    Atlantic (including geological constraints
    from plate reconstructions and rheological
    and crustal heterogenities).

   Parameter space analysis.

				
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posted:10/20/2012
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