Dynamics of continental accretion

Abstract

Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth’s history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides1, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia2. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4,5,6,7, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.

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Figure 1: Reconstruction of the Tasmanide accretionary event, showing indentation of continental ribbon material and lateral subduction.
Figure 2: Four frames in the evolution of model 80s taken at 18, 27, 39 and 61 Myr from the start of the simulation.
Figure 3: A comparison of velocities during collision, accretion and recovery.

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Acknowledgements

L.M. acknowledges support from the Australian Research Council’s Discovery Projects funding scheme (projects DP130101946 and DP110101697) and use of the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government. P.G.B. acknowledges the support of the Monash Research Accelerator Program. M.S.M. acknowledges the support of the National Science Foundation under grant no. EAR-1054638. R.C. publishes with the permission of the Director of the Geological Survey of Victoria. Underworld development was supported by AuScope, under the National Collaborative Research Infrastructure Strategy.

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Contributions

All authors contributed to the conception and writing of the paper and to the geodynamic analysis. L.M. and M.S.M. contributed the geophysical synthesis for the model set-up. P.G.B. and R.A.C. contributed the geological synthesis of the Tasmanides. L.M. ran the numerical models and made the animations.

Corresponding author

Correspondence to L. Moresi.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Initial geometry of the distinct material domains in the numerical model, not to scale.

The subducting plate is 100 km thick, 3,000 km wide and 7,000 km in length, and is built from 4 layers. The overriding plate has three domains: a back-arc region (1,200 km) a transitional region (350 km) and a continental backstop (750 km), each of which comprises two layers of equal thickness. The indenting ribbon is 50 km in thickness, 1,500 km wide and 500 km deep (perpendicular to the convergence direction). Each of the vertical boundaries is a symmetry plane.

Extended Data Figure 2 Viscosity, density and yield strength for each of the domains in the model shown in Extended Data Fig. 1.

a, 80-Myr-old lithosphere, and b, 120-Myr-old lithosphere, assuming a half-space cooling model; c, cratonic lithosphere, d, transitional lithosphere and e, the lithosphere of the back-arc region which assume a linear geotherm to the internal temperature. In the viscosity plots, the green line is the temperature and the blue line is the viscosity. The averaged, truncated value for each layer is shown by the horizontal bars. The black dashed line is the depth-dependent change in viscosity at the internal reference temperature normalized to the value at the base of the lithosphere. In the density plots, the solid line is the calculated density based on the temperature and composition and the bars are the relative density change from the density at the internal temperature. The strength plots show increasing strength with depth and pressure as separate lines and the actual strength used for each layer as a dark bar. After softening, the yield strength reduces as plastic strain increases following the function shown in f. The final yield strength is shown in the light coloured bars in each of the strength plots. In e there are two strength profiles shown, one for the ‘weak’ case (left) and one for the ‘strong’ case (right). Models 80w and 120w use the weaker profile in this region; models 80s and 120s use the stronger one.

Extended Data Figure 3 Comparative evolution of models 120s and 80w taken at corresponding times during the two runs.

Left column, 120s; right column, 80w. Colours and strain-rate scale are as used in Fig. 2.

Extended Data Figure 4 Comparative evolution velocities of model 120s (dashed lines) and 80w (solid lines) using the same colour key as Fig. 3.

The grey bars in the upper half of the diagram indicate the five frames in Extended Data Fig. 3 for model 80w, and the bars in the lower half indicate the times for model 120s.

Extended Data Figure 5 View of the arcuate slab in model 80w looking from above towards the laterally retreating trench.

Time after model start (in Myr): top, 15; middle, 24; bottom, 37. The upper plate has been made translucent to most clearly show the shape of the down-going plate and its relationship to the development of the orocline in the indenter. Reduced rollback rates in the convergence direction (Fig. 3) produce the folded slab geometry seen in the foreground. Two lines of markers have been traced along the strain-grid in the oceanic plate. They clearly show that the double-slab geometry is caused by foundering of the oceanic plate during lateral rollback. The trajectories of the individual points and the slab cross-section for the front of the pair of tracer-lines are shown in Extended Data Fig. 6.

Extended Data Figure 6 Material point trajectories for points on the central line of model 80w.

The points are sampled along a direction initially parallel to the plate-motion (right to left in this figure), which bisects the domain and passes along the short edge of the accreting ribbon. Colours (see key) indicate the times at which individual trajectories have been recorded. Inset, the plan view of the trench at time intervals indicated by the same colouring. Lateral motion of material points is negligible (<25 km in 650 km of vertical sinking). As is typical of rollback subduction, material trajectories are much steeper than the slab orientation (grey lines). The slab at the left-hand side hangs from the back of the accreted ribbon and undergoes slow stretching. Material in this hanging part of the slab has been all subducted in the right-to-left direction in the orientation of this diagram and only moves left-to-right during stretching.

Supplementary information

Model 80w (Table 1) side view from above the indenting ribbon

Evolution of the model with the 80 Myr old slab (grey with yellow grid) and weaker overriding plate (blue with light blue grid). The indenter is shown in red. The strain rate field close to the surface is superimposed on the colouring of the material domains to show high strain rates as red tones and intermediate strain rates in yellow/green. Snapshots of this video are shown in Extended Data Figure 3. (MOV 26234 kb)

Model 80s (Table 1) side view from above the indenting ribbon (version2)

As video 1, but with a stronger overriding plate as described in Table 1. Snapshots of this video are shown in Figure 2. (MOV 18453 kb)

Model 80s (Table 1) View from beneath the overriding plate

Evolution of the model with the 80Myr old slab viewed from beneath the overriding plate looking towards the incoming indenting ribbon. Colours are the same as described for video 1. The white/red balls are finite strain markers located under the oceanic plate at the start of the simulation. (MOV 28120 kb)

Model 120w (Table 1) side view from above the indenting ribbon

As video 1, but with a 120 Myr old plate. (MOV 16946 kb)

Model 120s (Table 1) side view from above the indenting ribbon

As video 1, but with a 120 Myr old plate and a stronger overriding plate as described in Table 1. Snapshots of this video are shown in Extended Data Figure 3. (MOV 24191 kb)

Model 120s (Table 1) View from beneath the overriding plate

As video 3 but with a 120 Myr old plate. (MOV 21051 kb)

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Moresi, L., Betts, P., Miller, M. et al. Dynamics of continental accretion. Nature 508, 245–248 (2014). https://doi.org/10.1038/nature13033

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