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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