Walter Alvarez (2010) argued that the protracted continental collisions of the Alps, Zagros, and Himalaya — where plate convergence persisted for tens of millions of years after buoyant continental lithosphere entered the subduction zone and the oceanic slab broke off — imply that continental plates are driven by basal traction from an asthenospheric 'undertow' rather than by slab pull. Fifteen years of work has sharpened every ingredient of this claim (slab break-off timing, tomography-based mantle flow, craton-root coupling, global convection modeling) but has produced directly conflicting quantifications of the force budget: 2024–2025 global models give mantle drag on the Indian cratonic root comparable to slab pull (Li et al. 2024) and slab-pull dominance with basal drag excluded beyond a secondary role (Zheng et al. 2025), while detachment-focused models find basal drag an order of magnitude weaker than slab pull (Wu et al. 2025). The open core is a precise, unresolved question: determine the quantitative partition between basal traction and slab pull for protracted continental convergence, and whether basal traction transmitted through or beneath continental roots can dominate and reproduce the first-order kinematics of the three type collisions.

Background

Plate motions are driven by a combination of forces whose relative sizes are still debated. Slab pull is the gravitational force of cold, dense subducted oceanic lithosphere sinking into the mantle and pulling its attached plate; ridge push is the gravitational push from elevated mid-ocean ridges; basal drag (basal traction) is the shear stress that mantle flow exerts on the base of a plate, which can either resist or drive plate motion depending on flow direction. During continental collision, buoyant continental crust resists subduction, and the oceanic slab commonly detaches (slab break-off), removing slab pull; yet in the Alps, Zagros, and Himalaya, convergence continued for tens of millions of years after collision — the observation on which the 'continental undertow' hypothesis is built. In this hypothesis (Alvarez, 1982, 1990), subduction-driven mantle return flow organizes into broad convergent flow axes beneath collision belts, and the asthenosphere drags the overriding continental plates with it like an undertow, sustaining convergence that edge forces alone supposedly cannot. Alvarez (2010) revived this idea and posed the question for the three classic Tethyan collisions. The mainstream quantification, however, has long been that slab pull dominates the plate-driving force budget and that basal drag is mostly resistive (Conrad and Lithgow-Bertelloni, 2002). A key mechanical unknown is the coupling between mantle flow and the thick, buoyant cratonic roots (keels of old, stiff lithosphere extending 150–250 km beneath continental cores): shear tractions grow where roots protrude into flowing asthenosphere (Conrad and Lithgow-Bertelloni, 2006), while any thin low-viscosity asthenospheric channel beneath the roots attenuates the coupling (Höink et al., 2011); the presence and thickness of such sub-root asthenosphere is hard to image. Whether mantle flow or slab pull organizes Tethyan tectonics has been argued from mantle-conveyor and orogeny-classification studies (Becker and Faccenna, 2011; Faccenna et al., 2013, 2021; Jolivet et al., 2018) and from spherical convection models of the plate-mantle force balance (Coltice et al., 2019). Discriminating tests need the timing of slab break-off (in the Zagros proposed by Molinaro et al., 2005, but contested by Agard et al., 2011), seismic tomography (3-D imaging of mantle velocity anomalies from earthquake waves) and seismic anisotropy (direction-dependent wave speed that records mantle flow fabric, e.g., from SKS splitting of core-traversing shear waves), and absolute plate reconstructions whose rotation-pole uncertainties grow with age.

Problem Statement

Determine the dominant driving mechanism of protracted continental convergence in the three type collisions of Alvarez (2010) — the Alps, the Zagros, and the Himalaya — by quantifying the force partition between (i) basal traction exerted by mantle flow on the bases of continental plates (the continental undertow), including its transmission through thick cratonic roots and any thin sub-root asthenosphere, and (ii) slab pull from attached or detached subducted lithosphere. The single overarching objective is an observationally constrained force budget that establishes whether basal traction can dominate convergence once slab break-off has removed or weakened slab pull, and that reproduces the observed convergence histories of the three collisions. Relevant constraints and evidence would include: the timing and spatial extent of slab break-off (notably in the Zagros); the presence, thickness, and mechanical role of sub-root asthenosphere and root–flow coupling; and the deep mantle circulation pattern (from seismic tomography and anisotropy) that would supply the traction — all within the limits set by uncertainties in absolute plate reconstructions, which grow with age.

References

  1. Alvarez, W. (1982). Geological evidence for the geographical pattern of mantle return flow and the driving mechanism of plate tectonics. Journal of Geophysical Research: Solid Earth 87, 6697–6710. DOI 10.1029/JB087iB08p06697. https://doi.org/10.1029/JB087iB08p06697
  2. Alvarez, W. (1990). Geologic evidence for the plate-driving mechanism: the continental undertow hypothesis and the Australian–Antarctic Discordance. Tectonics 9, 1213–1220. DOI 10.1029/TC009i005p01213. https://doi.org/10.1029/TC009i005p01213
  3. Alvarez, W. (2010). Protracted continental collisions argue for continental plates driven by basal traction. Earth and Planetary Science Letters 296, 434–442. DOI 10.1016/j.epsl.2010.05.030. https://doi.org/10.1016/j.epsl.2010.05.030
  4. Conrad, C. P.; Lithgow-Bertelloni, C. (2002). How mantle slabs drive plate tectonics. Science 298, 207–209. DOI 10.1126/science.1074161. https://doi.org/10.1126/science.1074161
  5. Conrad, C. P.; Lithgow-Bertelloni, C. (2006). Influence of continental roots and asthenosphere on plate–mantle coupling. Geophysical Research Letters 33, L03312. DOI 10.1029/2005GL025621. https://doi.org/10.1029/2005GL025621
  6. Höink, T.; Jellinek, A. M.; Lenardic, A. (2011). Viscous coupling at the lithosphere–asthenosphere boundary. Geochemistry, Geophysics, Geosystems 12, Q06002. DOI 10.1029/2011GC003698. https://doi.org/10.1029/2011GC003698
  7. Molinaro, M.; Zeyen, H.; Laurencin, X. (2005). Lithospheric structure beneath the south-eastern Zagros Mountains, Iran: recent slab break-off? Terra Nova 17, 1–6. DOI 10.1111/j.1365-3121.2004.00575.x. https://doi.org/10.1111/j.1365-3121.2004.00575.x
  8. Agard, P.; Omrani, J.; Jolivet, L.; Whitechurch, H.; Vrielynck, B.; Spakman, W.; Monié, P.; Meyer, B.; Wortel, R. (2011). Zagros orogeny: a subduction-dominated process. Geological Magazine 148, 692–725. DOI 10.1017/S001675681100046X. https://doi.org/10.1017/S001675681100046X
  9. Becker, T. W.; Faccenna, C. (2011). Mantle conveyor beneath the Tethyan collisional belt. Earth and Planetary Science Letters 310, 453–461. DOI 10.1016/j.epsl.2011.08.021. https://doi.org/10.1016/j.epsl.2011.08.021
  10. Faccenna, C.; Becker, T. W.; Conrad, C. P.; Husson, L. (2013). Mountain building and mantle dynamics. Tectonics 32, 80–93. DOI 10.1029/2012TC003176. https://doi.org/10.1029/2012TC003176
  11. Jolivet, L.; Faccenna, C.; Becker, T.; Tesauro, M.; Sternai, P.; Bouilhol, P. (2018). Mantle flow and deforming continents: from India–Asia convergence to Pacific subduction. Tectonics 37, 2887–2914. DOI 10.1029/2018TC005036. https://doi.org/10.1029/2018TC005036
  12. Coltice, N.; Husson, L.; Faccenna, C.; Arnould, M. (2019). What drives tectonic plates? Science Advances 5, eaax4295. DOI 10.1126/sciadv.aax4295. https://doi.org/10.1126/sciadv.aax4295
  13. Faccenna, C.; Becker, T. W.; Holt, A. F.; Brun, J.-P. (2021). Mountain building, mantle convection, and supercontinents: revisited. Earth and Planetary Science Letters 564, 116905. DOI 10.1016/j.epsl.2021.116905. https://doi.org/10.1016/j.epsl.2021.116905
  14. Li, Y.; Liu, L.; Li, S.; Peng, D.; Cao, Z.; Li, X. (2024). Cenozoic India-Asia collision driven by mantle dragging the cratonic root. Nature Communications 15, 6674. DOI 10.1038/s41467-024-51107-0. https://doi.org/10.1038/s41467-024-51107-0
  15. Koshnaw, R. I.; Kley, J.; Schlunegger, F. (2024). The Miocene subsidence pattern of the NW Zagros foreland basin reflects the southeastward propagating tear of the Neotethys slab. Solid Earth 15, 1365–1383. DOI 10.5194/se-15-1365-2024. https://doi.org/10.5194/se-15-1365-2024
  16. Wu, Y.; Faccenda, M.; Liao, J.; Fan, J.; Guo, F.; Lai, Z. (2025). Mantle-driven plate convergence due to slab detachment. Communications Earth & Environment 6, 494. DOI 10.1038/s43247-025-02484-x. https://doi.org/10.1038/s43247-025-02484-x
  17. Govers, R.; Herman, M. W.; van de Wiel, L.; Nijholt, N. (2025). Probabilistic assessment of the causes of active deformation in Greece, Western Anatolia, and the Balkans using finite element models. Tectonics 44, e2024TC008658. DOI 10.1029/2024TC008658. https://doi.org/10.1029/2024TC008658
  18. Zheng, Q.; Hu, J.; Gurnis, M.; Chen, L.; Shi, Y.; Bao, X.; Yang, Y. (2025). Ongoing India–Eurasia collision predominantly driven by Sumatra–Java slab pull. Nature Geoscience 18, 909–915. DOI 10.1038/s41561-025-01771-8. https://doi.org/10.1038/s41561-025-01771-8
  19. Straume, E. O.; Faccenna, C.; Becker, T. W.; Steinberger, B.; Licht, A.; Sembroni, A.; Gvirtzman, Z.; Ballato, P. (2025). Collision, mantle convection and Tethyan closure in the Eastern Mediterranean. Nature Reviews Earth & Environment 6, 299–317. DOI 10.1038/s43017-025-00653-2. https://doi.org/10.1038/s43017-025-00653-2