Deciphering the Surface Expressions of Subduction-Driven Mantle Flow

This review article examines the complex interplay between subduction dynamics, mantle flow, and their resulting expressions on Earth’s surface. The authors emphasize the importance of understanding mantle flow in 3D, moving beyond the limitations of 2D representations to account for the Earth’s spherical geometry and the intricate nature of subduction zones.

A diagram of a river Description automatically generated
Subduction-induced mantle flow patterns. Poloidal flow (blue arrows) is active both in front of and behind the slab, whereas toroidal flow (orange arrows) occurs around the slab edges. Forces that act on the subducting and overriding plate are shown: driving forces (white arrows) include ridge push (Frp), slab pull (Fsp), negative buoyancy of the subducting lithosphere (Fnb) and trench suction (Fts); and resisting forces (black arrows) include mantle drag (Rd.c/Rd.o), resistance at the subduction interface (Rs-c), bending resistance (Rb), mantle resistance on the slab (Rs) and mantle resistance on the ridge (Rr).

Key Concepts

  • Subduction-Induced Mantle Flow: The sinking of oceanic plates (subduction) triggers two distinct patterns of mantle flow:
    • Poloidal flow, often simplified as corner flow, operates in the vertical plane, drawing new asthenospheric mantle into the mantle wedge.
    • Toroidal flow, occurring in the horizontal plane, is driven by the lateral movement of slabs, frequently associated with slab rollback, diverting mantle material from behind the slab around its edges and into the mantle wedge.
  • Surface Expressions: The authors highlight the various ways in which mantle flow patterns are manifested on Earth’s surface:
    • Dynamic Topography: Mantle flow, particularly in the upper mantle, exerts viscous coupling on the overlying lithosphere, transferring stresses and contributing to dynamic changes in surface topography. The authors emphasize that while mantle flow influences topography, its distinct contribution is often challenging to isolate due to the interplay of crustal and lithospheric processes at convergent margins.
    • Uplift and Subsidence:
      • Upwelling of asthenospheric mantle, often associated with toroidal flow around slab edges or through slab tears and holes, can cause dynamic uplift. For example, the uplift observed around Mount Etna in the Calabrian subduction zone is partly attributed to mantle flow around the slab edge.
      • Conversely, the initiation of flat slab subduction can lead to subsidence as the dense slab flattens against the overlying plate.
    • Erosion and Exhumation: Changes in dynamic topography induced by mantle flow can influence erosion and exhumation rates, providing clues to the long-term topographic evolution.
    • Interaction with Other Processes: The authors acknowledge that deciphering the specific influence of mantle flow on topography requires careful consideration of other contributing factors, including:
      • Isostatic Adjustment: Isostatic responses to changes in crustal thickness, loading, and unloading can significantly affect topography. For instance, the uplift observed following the termination of flat slab subduction is primarily attributed to isostatic rebound.
      • Lithospheric Deformation: Tectonic processes such as shortening, extension, and faulting can contribute to topographic changes. The formation of mountain ranges and basins is often the result of complex interactions between mantle flow, lithospheric deformation, and isostatic adjustment.
Mantle flow around subducting slabs and its associated surface expressions. a, Along-trench variations. Distinct geochemical signatures can be observed in along-trench mantle flow only if they are present in the first place. b, Slab rotation. The location of different tectonic styles associated with slab rotation can vary. c, Flat-slab subduction. Dashed lines show the geometry of the slab and the mantle flow at a previous stage in the formation (left) and termination (right) of flat-slab subduction. Uplift in the formation phase and subsidence in the termination phase are not directly caused by mantle flow but, rather, by the depth and location of the slab. d, Slab hole. Surface expressions of slab holes might only be observed if the hole is at a shallow depth and the resulting upwelling of asthenospheric mantle occurs close to the surface. Orange arrows represent expected mantle flow strength and direction; black arrows represent the trench retreat rat
  • Specific examples of the surface expressions of subduction-driven mantle flow include:
    • The Tonga subduction zone, where the Samoan plume material is entrained by toroidal flow, leading to distinct geochemical signatures in the Lau Basin basalts.
    • The Calabrian subduction zone, where toroidal flow around the southern edge of the slab, coupled with mantle upwelling, is thought to be responsible for the intraplate-like volcanism of Mount Etna and the uplift observed in the region.
    • The Central American subduction zone, where geochemical gradients and seismic anisotropy data suggest northward trench-parallel flow, potentially caused by variations in slab dip angle and trench retreat velocity.
    • Flat slab subduction zones, where the flattening of the slab can lead to migration and cessation of arc volcanism, the formation of small-scale convection cells, and potentially localized mantle upwelling and volcanism.
    • Slab tears and holes, which create pathways for mantle flow, leading to altered subduction geometry, trench kinematics, and potentially dynamic uplift and volcanism if the hole is sufficiently large and shallow.
  • Dynamic Variations: Subduction zones are dynamic systems with various factors influencing mantle flow patterns.
    • Along-trench variations in subduction parameters, such as slab dip angle and trench velocity, can lead to complex 3D flow patterns. Geochemical gradients in volcanic rocks and seismic anisotropy patterns can help track these along-trench variations, as seen in the Central American subduction zone.
    • Slab tears and holes create pathways for mantle flow, altering subduction geometry, trench kinematics, and potentially inducing uplift and volcanism if the holes are large and shallow enough. The Pampean flat slab in South America, with its prominent slab hole, exemplifies these effects.
    • Interactions between neighboring subduction zones, especially when in close proximity, can influence mantle flow patterns, leading to changes in subduction dynamics and surface expressions. The Mediterranean region, with its complex tectonic history and multiple interacting slabs, demonstrates the effects of such interactions.

Understanding Mantle Flow in 3D

The most significant contribution of this papers is the emphasis by the authors that while it is convenient to simplify mantle flow processes as 2D, the Earth’s spherical nature and the complexity of subduction dynamics necessitate a 3D understanding of mantle flow.

  • The traditional conceptualization of poloidal flow as corner flow is often disrupted by 3D effects. This means that the asthenospheric mantle entering the mantle wedge can come from a broader region than anticipated in 2D models.
  • Toroidal flow is a crucial component of mantle dynamics, often occurring around the edges of slabs, particularly during slab rollback.
  • Understanding toroidal flow in 3D is vital to explain phenomena like the curved geometry of trenches and slabs, along-trench variations in trench retreat rates, and the interaction of neighboring subduction zones. For example, toroidal flow can lead to faster trench retreat in the center of narrow subduction zones, resulting in a concave trench shape. Conversely, in wide subduction zones, toroidal flow may only be prominent near the edges, leading to slower retreat in the center and a convex trench shape.
  • 3D effects are also essential to consider when interpreting observations of mantle flow. For instance, seismic anisotropy data, often used to infer mantle flow directions, can be complex and challenging to interpret, especially regarding the depth of the detected anisotropy.
  • Distinguishing between flow in the mantle wedge and the sub-slab mantle can be difficult.
  • Understanding the 3D nature of these flow patterns is crucial for accurately interpreting seismic anisotropy data and reconstructing mantle flow patterns.

Author

Leave a Reply