Subduction erosion: rates, mechanisms, and its role in arc magmatism and the evolution of the continental crust and mantle

Charles R. Stern¹

¹Department of Geological Sciences, University of Colorado, Boulder, Colorado, 80309-0399 USA, Charles.Stern@colorado.edu

This webpage is a summary of: Stern, C.R. (2011). Subduction erosion: Rates, mechanisms, and its role in arc magmatism and the evolution of the continental crust and mantle, Gondwana Research, doi:10.1016/j.gr.2011.03.006.

Subduction erosion occurs at all convergent plate boundaries, even if they are also accretionary margins. Frontal subduction erosion results from a combination of erosion and structural collapse of the forearc wedge into the trench. Basal subduction erosion results from abrasion and hydrofracturing above the subduction channel (Figure 1).

Figure 1. Cross-section, modified from von Huene et al. (2004), illustrating the components of the forearc wedge and different processes involved in subduction erosion. The subduction channel initially is filled with both oceanic sediment and debris eroded off the forearc wedge surface that accumulates in the frontal prism. Basal erosion results in mass transfer from the bottom of the forearc wedge to the lower plate as dislodged fragments are dragged into the subduction channel. As pore fluid is lost from the sediments in the channel, the strength of coupling between the two plates increases and the seismogenic zone begins.

High rates of subduction erosion are associated with relatively high convergence rates (> 60 mm/yr) and low rates of sediment supply to the trench (< 40 km²/yr). This implies a narrow and topographically rough subduction channel which is neither smoothed out nor lubricated by fine-grained water-rich turbidites such as are transported into the mantle below accreting convergent plate boundaries. Rates of subduction erosion may be > 440 km³/km/my. They vary temporally as a function of these same factors, and because of the subduction of buoyant features such as seamount chains, volcanic plateaus, island arcs and oceanic spreading ridges, which weaken the forearc wedge.

Some revised estimates of long-term rates of subduction erosion are:

• SW Japan (≥ 30 km³/km/my since 400 Ma);
• SW USA (≥ 30 km³/km/my since 150 Ma);
• Peru and northern Chile (50-70 km³/km/my since 150 Ma);
• central (115 km³/km/my since 30 Ma);
• southernmost Chile (30-35 km³/km/my since 15 Ma).

These are higher than in previous compilations (Stern, 2011).

Globally, subduction erosion is responsible for > 1.7 Armstrong Units (1 AU = 1 km³/yr) of crustal loss, 33% of the ~ 5.25 AU of yearly total crustal loss. This is more than any of either sediment subduction (1.65 AU), continental lower crustal delamination (≥ 1.1 AU), crustal subduction during continental collision (0.4 AU), or subduction of rock-weathering-generated chemical solute dissolved in oceanic crust (0.4 AU). The paucity of pre-Neoproterozoic blueschists suggests that global rates of subduction erosion were probably greater in the remote past, perhaps due to higher plate convergence rates.

Subducted sediments and crust removed from the over-riding forearc wedge by subduction erosion may remain in the crust, either underplated below the wedge, or carried deeper into the source region of arc magmatism. There, they may be incorporated into arc magmas via either dehydration of the subducted slab and the transport of their soluble components into the overlying mantle wedge, and/or bulk melting of the subducted crust to produce adakites. In selected locations such as Chile, Costa Rica, Japan and SW USA, there are temporal and spatial correlations between the crustal isotopic characteristics of arc magmas and episodes or areas of enhanced subduction erosion.

Nevertheless, most subducted crust and sediment (> 90%, i.e., > 3.0 AU) is transported deeper into the mantle and neither underplated below the forearc wedge nor incorporated in arc magmas. The total current rate of return of continental crust into the deeper mantle, the most important process being subduction erosion, is equal to or greater than the rate at which the crust is being replaced by magmatic activity. This indicates that currently, the continental crust is probably shrinking slowly. Rates of crustal growth may have been episodically more rapid in the past, most likely at times of supercontinent breakup. Conversely, rates of crustal destruction may have also been higher during times of supercontinent amalgamation (Stern & Scholl, 2010; Santosh, 2010).

The supercontinent cycle thus controls the relative rates of growth and destruction of the continental crust. Subduction erosion plays an important role in producing and maintaining this cycle by transporting radioactive elements from the crust into the mantle. Geochemical studies on intraplate and "hotspot" lavas report trace-element and isotopic evidence for a component of recycled sediment and/or continental crust. This could thus be explained by sediment and/or forearc crust subducted into the mantle by subduction erosion, contributing to the melt source.

References

• Santosh, M., 2010. A synopsis of recent conceptual models on supercontinent tectonics in relation to mantle dynamics, life evolution and surface environment. Journal of Geodynamics 50, 116–133.
• Stern, C.R., Subduction erosion: Rates, mechanisms, and its role in arc magmatism and the evolution of the continental crust and mantle, Gondwana Research (2011), doi:10.1016/j.gr.2011.03.006
• Stern, R.J., Scholl, D.W., 2010. Yin and yang of continental crust creation and destruction by plate tectonic processes. International Geology Review 52, 1–31.
• von Huene, R., Ranero, C., Vannucchi, P., 2004. Generic model of subduction erosion. Geology 32, 913–916.