Analysing the role of ML scars on tectonics

Figure 2a shows 11 differently angled weak zones in both UC and LC, and one ML scar. We then compare convergent (1 cm per year) models after a significant amount of shortening (350–450 km) that feature only UC scars (Fig. 2b), LC scars (Fig. 2c), UC and LC scars (Fig. 3a), and all the available weak zones (Fig. 3b).

Figure 2: Model set-up and results with UC and LC weak zones. (a) Model set-up (red: UC (wet quartzite); green: LC (Maryland diabase); yellow: ML (dry olivine)). The top 150 km of the 600-km deep model is shown at the initial condition (t=0 Myr) with the configuration of UC, LC and ML weak scars. The full width of the model is shown. Continental convergence is incorporated by introducing new lithosphere at the right boundary of box with velocity =1 cm per year. (b). Material deformation (top) and visualization of the second invariant of the deviatoric strain rate tensor (bottom) after 450 km of shortening for only UC weak scars. (c) As b, but for LC weak scars only. The alphabetical markers (A–G) show the progression of the deformation throughout the full model simulation and track the evolution of tectonics. Full size image

Figure 3: Model results for combinations of weak scars. (a) Material deformation (top) and visualization of the second invariant of the deviatoric strain rate tensor (bottom) after 450 km of shortening for a combination of UC and LC weak scars. Markers highlight the order in which deformation occurs. (b) As a, but including a ML weak scar (configuration given in Fig. 2a). (c) Deformation for only ML weak zone. The alphabetical markers (H–K) show the progression of the deformation throughout the full model simulation and track the evolution of tectonics. Where only one alphabetical marker is shown (L and M), the deformation does not move through the full simulation. Dashed lines and markers L and M correspond to the width of deformation after 350 km of shortening. Full size image

The reactivation of crustal scars is time-dependent, with shifting strain patterns occurring once a fault has localized over time18. The time-dependence of the UC scars is shown in Fig. 2b by markers A, B and C, with these faults reactivating in alphabetical sequence as strain is accumulated and transferred. The UC fault reactivation produces folding in the UC, but acts to thicken the LC and produce subcrustal subduction within the ML. When only LC scars are present (Fig. 2c), a time-dependent strain pattern change remains (markers D–G) but the thickening of the LC produces widespread folding in the UC.

The combination of both LC and UC scars (Fig. 3a) produces a shifting pattern of strain that appears to be controlled by the lower crustal heterogeneities rather than the shallower features. Timing of crustal fault ruptures (H–K) coordinate with those of LC scars only (Fig. 2c). Furthermore, the combination of LC and UC faults produces a strong decoupling of the LC from the UC, as shown by the high strain rate along the boundary.

The introduction of a ML scar to convergent models featuring 22 remnant crustal features produces a marked focusing of deformation (Fig. 3b). The ML scar supersedes the shallower pre-existing faults to produce ‘pseudo-subduction’ (that is, underthrusting of the ML). Lower crustal thickening impinges on the UC to produce folding. Over time, the folding propagates as the convergence continues. However, the deformation produced by the ML scar is localized (despite the numerous crustal features present) with little time-dependence. In fact, the pattern of deformation produced by UC, LC and ML weak scars (Fig. 3b) is almost identical to that of a convergent model featuring a ML scar only (Fig. 3c). Subsequently, the evolution of intraplate deformation follows the later stages of a continent collision (for example, ref. 19).

Model validation

Additional experiments were performed to test how robust the model set-up is to changes in rheological parameters. Supplementary Fig. 2 shows the strength profiles for the rheology used in Figs 2 and 3 (Supplementary Fig. 2a), as well as for the additional experiments (Supplementary Fig. 2b–d). The models presented in Supplementary Figs 3 and 4 show ML scars to be important in activating tectonics, while the reduction in crust and mantle strength may also determine the dominant factor in tectonic evolution. The models presented in Figs 2 and 3 are the most relevant as we apply a strong LC and ML to simulate relatively stable, strong continental interiors that feature long-lived ML scars. We present additional models here to understand the limits of our study.

Jelly sandwich or crème brûlée

Supplementary Fig. 3 shows results from using a crust and ML rheology that is not as strong as those experiments presented in Figs 2 and 3. We implement a different configuration of weak zones for this suite of models (Supplementary Fig. 3a) as compared with Fig. 2a, with 12 weak zones in the LC and UC and 2 in the ML. We acknowledge that previous studies have highlighted the possible ‘jelly sandwich’ or ‘crème brûlée’ strength profiles of the mantle (for example, ref. 13), and present experiments taking this into account in Supplementary Fig. 3. Our jelly sandwich experiments have a weaker LC than shown in Figs 2 and 3 (strength profile as given in Supplementary Fig. 2b). For models just featuring UC scars, we find that the UC dominates tectonic evolution in this jelly sandwich setting (Supplementary Fig. 3b). The time markers A–C show the evolution of strain within the UC.

The introduction of weak zones into the LC and ML (Supplementary Fig. 3c) generates more localized deformation. Although the UC weak zones are reactivated, their influence on regional tectonics and the deformation of the LC is minimal. ML scars again show to be dominant in activating tectonics.

In applying a crème brûlée rheology, we detach the LC and mantle from the UC leading to décollment tectonics (for example, ref. 13). Our results indicate that regions where a crème brûlée rheology is present, perennial plate tectonics may not persist. However, in order for stable continents to be preserved over time, Burov and Watts13 found that a jelly sandwich rheology would be required over a crème brûlée rheology. As a result, the crème brûlée' rheology is not applicable to our study of intraplate tectonics.

LC strength

The strength of the LC and ML is important in activating tectonics (for example, refs 14, 18, 20, 21). To show that our results are not dependent on the rheological parameters, we present a suite of models that use material values that are similar to a previously published paper22. In these models we use the weak zone configuration as shown in Supplementary Fig. 3a, and implement flow laws for wet quartz for the UC23, wet anorthite for the LC24 and dry olivine for the upper mantle25 (Supplementary Table 2). These rheological parameters produce strength profiles that are intermediate in the range of published flow laws for crustal and mantle materials (Supplementary Fig. 2d).

Supplementary Fig. 4 shows the material deformation after 270 km of shortening for the rheological parameters given in Supplementary Table 2. The UC weak zones in Supplementary Fig. 4a generate strong folding of the crust. The changing patterns of stress build-up shifts to produce pseudo-subduction of the crust into the ML. The implementation of wet anorthite for the LC generates a layer that is not primed for failure (even with regions of designated weakness). The wet anorthite flow law generates a weak LC that plays no role in the deformation pattern. This is shown with the introduction of UC and LC weak zones (Supplementary Fig. 4b) and the dominance of the upper crustal weak zones in the deformation pattern.

The ML weak zones again dominate the deformation pattern when introduced in Supplementary Fig. 4c. Although the UC weak zones reactivate, the overall tectonic pattern is controlled by the ML scars (with the LC weak zones playing no role). The time-dependent nature of the UC deformation pattern is replaced by the ML scarring’s ability to accommodate shortening through sub-crustal subduction. In an experiment featuring only ML scars (Supplementary Fig. 3d), the deformation is localized. However, the pattern of crustal deformation with all weak zones present (Supplementary Fig. 4c) follows closely with that of the simulation with just ML weak zones (Supplementary Fig. 4d), rather than that of UC only (Supplementary Fig. 4a), implying that the deeper scars control tectonic evolution.

Plate velocity

By increasing the convergence rate of the model from =1 to 2.5 cm per year, we can show the difference in deformation due to plate forcing. Here we compare simulations that have been shortened by 450 km using the rheological parameters of Supplementary Fig. 2a and Supplementary Table 1 (and the weak zone configuration shown in Fig. 2a). The only major difference that occurs from increasing the convergence velocity (when comparing with Fig. 2) happens for the model featuring UC scars only (Fig. 2b and Supplementary Fig. 5a). At a lower convergence velocity the stress pattern changes over time reactivating faults over time (Fig. 2b and Supplementary Fig. 2b). However, at =2.5 cm per year the stress pattern does not change and the time-dependence of the model disappears. In the presence of a higher convergence velocity, the result that lower crustal weak scars control deformation over shallower features is highlighted due to this lack of time-dependence with UC scars. The deformation pattern from UC and LC scars (Supplementary Fig. 5c) is more similar to LC tectonics (Supplementary Fig. 5b) than UC tectonics (Supplementary Fig. 5a). Overall, increasing the convergence velocity allows for the same conclusions: deep lithospheric heterogeneities can control deformation over shallower features.

ML weak zone orientation

Supplementary Fig. 6 give an indication of how the orientation of a ML weak scar would affect the pattern of deformation. In all previous models we applied similar angles to the scars within the ML. Supplementary Fig. 6a gives four different ML scar configurations: ML(i) two scars with angles as previously shown; ML(ii) two steeper ML scars; ML(iii) two vertical ML scars; and ML(iv) two smaller ML scars (of the same size as the lower crustal scars). All models are realized with UC and LC scars as in Supplementary Fig. 3a. Models ML(i), ML(ii) and ML(iv) exhibit very similar deformation patterns despite having different configurations. In a result shown here, we found that if a ML scar becomes small enough, the crustal scars can control deformation.

The vertical nature of the weak scars in model ML(iii) inverses the deformation pattern, which indicates that there appears to be a preferred angle in which the ML can pseudo-subduct. However, the overall result from changing the orientation of the weak scars in the ML is that deeper heterogeneities often controls the pattern of deformation (despite changing the angles of scars).