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Modeling mantle dynamics of oceanic triple junctions: Effects of variable viscosity

Modeling mantle dynamics of oceanic triple junctions: Effects of variable viscosity. Jennifer Georgen Department of Geological Sciences Florida State University Acknowledgments: Jian Lin Woods Hole Oceanographic Institution. What is a triple junction?.

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Modeling mantle dynamics of oceanic triple junctions: Effects of variable viscosity

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  1. Modeling mantle dynamics of oceanic triple junctions: Effects of variable viscosity Jennifer Georgen Department of Geological Sciences Florida State University Acknowledgments: Jian Lin Woods Hole Oceanographic Institution

  2. What is a triple junction? • Triple junction: A special type of plate boundary where three plates meet at a single point. • There are three types of plate boundaries: • Ridge (R) • Transform fault (F) • Subduction zone or trench (T) • Sixteen types of triple junctions exist in theory (e.g., RRR, RTF, TFF) (McKenzie and Morgan, 1969). USGS

  3. What is a triple junction? • Triple junctions are either stable or unstable: • Stable: Exist for long periods of geologic time without evolving into a different configuration. • Unstable: Exist only for a geologic moment before evolving to a different configuration. • This study focuses on RRR triple junctions: • Only configuration that is unconditionally stable for all plate boundary orientations (azimuths) and relative motions. • Easiest to model.

  4. Objectives • To investigate three-dimensional patterns of mantle flow and temperature beneath oceanic ridge-ridge-ridge triple junctions. • Spreading rate • Background asthenospheric flow • Variable viscosity

  5. RRR Triple Junctions

  6. Bouvet TJ Galapagos TJ Azores TJ Rodrigues TJ RRR Triple Junctions

  7. BTJBTJ RRR Triple Junctions, Simplified

  8. -1000 -750 -500 -250 0 km 250 500 750 1000 -1000 -750 -500 -250 0 km 250 500 750 1000 ModelSet-Up • 3D finite element model • Triple junction fixed in middle of model box • Top B.C. = plate motion • Isoviscous (passive model) • Top 0°C, bottom 1350°C

  9. Upwelling Patterns

  10. Temperature Upwelling Rate

  11. ConstrainingModelPredictionsAgainstBathymetry Georgen and Lin, 2002

  12. BackgroundAsthenospheric Flow Along-axis: 600 km (33 nodes, min spacing 6 km) Across-axis: 300 km (25 nodes, min spacing 8 km) Depth: 200 km (13 nodes, min spacing 5 km) Bottom boundary condition: Model-predicted upper mantle flow conditions Plate-driven and density-driven flow (Behn et al. 2004)

  13. BackgroundAsthenospheric Flow Bottom boundary condition: Absolute plate motion (Gripp and Gordon 2002)

  14. BackgroundAsthenospheric Flow

  15. ho = 1019 Pa s, Qo = 260 kJ/mol (hmax = 1022 Pa s) Variable Viscosity h = ho exp (Qo/RT - Qo/RTm)

  16. Conclusions Isoviscous Slowest-spreading branch: • Large component of along-axis flow directed away from triple junction • Upwelling velocity and temperature are predicted to increase significantly within a few hundred km of the triple junction, to match those of the fastest-spreading branch Fastest-spreading branch: • Thermal and velocity fields are similar to single ridge case Background Asthenospheric Flow • Little difference in subaxial temperature patterns compared for isoviscous case Variable Viscosity • Increases subaxial temperatures and upwelling velocities • Narrows zone of fastest upwelling. • Significant axial increases ~50 km from triple junction.

  17. Bouvet TJ Galapagos TJ Azores TJ Rodrigues TJ RRR Triple Junctions

  18. OtherTripleJunctions Azores-like Rodrigues-like Galapagos-like

  19. OtherTripleJunctions

  20. BackgroundAsthenospheric Flow

  21. BackgroundAsthenosphericFlow Axis-parallel profile 60 km off-axis, 30 km depth, African plate Axis-parallel profile 60 km off-axis, 30 km depth, Antarctic plate

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