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Three fates of material on the subducting plate

goal: identify and explore key factors controlling the development of mountain belts through the lens of the critical taper concept qualitatively review critical taper concept compare models with convergent margins what controls underplating?. Three fates of material on the subducting plate

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Three fates of material on the subducting plate

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  1. goal: identify and explore key factors controlling the development of mountain belts through the lens of the critical taper concept • qualitatively review critical taper concept • compare models with convergent margins • what controls underplating?

  2. Three fates of material on the subducting plate • frontal accretion- addition of material at the toe of a wedge • underplating- addition of material to the base of a wedge (generally by development of a duplex) • subduction

  3. Wedges of the World ~100 km vertically exaggerated accretionary and nonaccretionary plate margins, Clift & Vannucchi, 2004 Some other famous orogenic wedges, same horizontal scale as above

  4. Wedge shape is maintained by feedbacks attempting to restore a balance of forces Davis et al, 1983

  5. Consider a segment whose taper angle became smaller (subcritical) Davis et al, 1983 Driving stress (σx difference) is reduced, resulting net force in the –x direction results in compression of the segment. i.e. taper is too low to overcome frictional stresses and segment travels with the subducting plate until taper is restored.

  6. Example of two transects through the Bolivian Andes, high erosion northern region and low-erosion southern region Horton, 1999

  7. low decollement strength or strong wedge  lower taper Tarim Basin 200 km Hotian area

  8. Simple critical taper model consistent with common observation of increasing metamorphic grade toward hinterland early computer models also predicted distributed deformation... Barr & Dahlen, 1990

  9. Complexities, using Taiwan as a case study Hundreds of data points collected in the past 15 years using half a dozen low-temperature thermochronometers and thermometers quantify heterogeneous erosion rates and kinematic history of Taiwan Simões et al, 2005

  10. Some differences: -material added by underplating/duplexing as well as by frontal accretion -heterogeneous deformation in wedge -ramp-flat geometry -decollement shifts Barr & Dahlen, 1990 vs. or Taiwan x-section from Yue, Suppe, Hung, 2005 Taiwan model, Simões et al, 2005

  11. Lesser Himalaya duplex Underplating • In Taiwan Barr & Dahlen estimated <10-25%, Simões et al estimate ~70% • Major feature of many orogenic wedges • Facilitates non-homogeneous deformation • Crustal budget implications Nepalese Himalaya, Avouac, 2003 Sevier FTB, DeCelles & Mitra, 1995 Taiwan x-section from Yue, Suppe, Hung, 2005

  12. Sandbox modeling

  13. Insights on underplating from sandbox modeling Parameters Erosion? No Weak layer? No Features -Develops critical taper -Dominated by frontal accretion Davis et al, 1983

  14. Insights on underplating from sandbox modeling Parameters Erosion? No Weak layer? No Higher basal friction Features -Taper angle alternates -Frontal accretion and underplating alternate (this model not at steady state condition) -underplating occurs predominantly by underthrusting of long sheets, not by duplexing Gutscher et al, 1996

  15. Varying parameters • Variation of wedge parameters is inevitable • Wedges (or portions of them) will shift between stable and subcritical • Finite element modeling by Willett (1992) suggests • Increased basal strength leads to internal wedge deformation • Decreased basal strength focuses deformation at the wedge toe DeCelles & Mitra, 1995

  16. Insights on underplating from sandbox modeling Parameters Erosion? Yes Weak layer? Yes Features -Nice duplex -Fairly realistic geometry 115 cm convergence 68 cm Movie... 21 cm convergence Konstantinovskaia and Malavieille, 2005

  17. Comparison of model with Himalaya

  18. Take home message High erosion rates suggest feedback between erosion and duplexing. A similar relationship between erosion and duplexing may be occurring in Taiwan (Simões et al, 2003) • Erosion probably plays a key role in the formation of duplexes Avouac, 2003

  19. Conclusions and Questions • Orogenic wedges may cycle through stages of stable and subcritical, remaining stable or subcritical for some time • Presence of weak layers plays a key role in developing realistic looking models in the sandbox • Erosion plays a key role in the formation of duplexes (predicts difference between subaqueous and exposed wedges) Questions • How to put critical taper to use in a more quantifiable manner?

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