Modes of collisional orogenic growth: forward, backward and thermally induced

When two continents collide different surface expressions can be produced. Triangular wedges are relatively narrow, while plateaus are high topographic features extending over large areas. Several studies have focused on the transition from wedges to plateaus, but the dynamic conditions of their growth remain elusive. Although different models for orogenic growth have been proposed, the link between theoretical/experimental models and natural analogues proves to be an outstanding task yet to be resolved.

Here, we present 2D high resolution (2048 x 512) buoyancy-driven numerical models, coupled with density phase diagrams, of sustained continental collision and subduction. We explore how crustal rheology controls the development of different types of orogenic growth and their subsequent final orogenic architecture, while further benchmarking our results to natural analogues.

Our results show that continental subduction can be sustained without the need for external forces and that three types of orogenic growth modes can be identified: i) forward; ii) backward; and iii) thermally induced. We show that the different types of orogenic growth are highly dependent on crustal rheology that, under high stresses, can allow large-scale lower crustal detachments to be formed and delamination processes to be developed. 

For weak lower crust rheologies, our results always show the development of a lower crustal detachment that connects both continents. In turn, subducting crustal material is thrusted onto the overriding continent, leading to compression of the two continents. In this case, a progressive uplift of the orogen in direction of the overriding continent is observed (forward orogenic growth). 

For a strong lower crust, no large-scale lower crustal detachment connecting both continents is formed. As such, the incoming crustal material is progressively stacked at the collision zone and the deformation is propagated backwards. Thus, the orogen continuously grows in direction of the subducting continent (backward orogenic growth). 

However, backward orogenic development can only occur over large periods of time if the strength of the subducting continental crust is sufficiently low to sustain continuous deformation of the crustal material. While a weak upper crust enables steady backward orogenic growth, a strong upper crust halts continental subduction and collision. Due to a stronger upper crust, the slab pull force is not sufficient to continuously deform the crustal material while maintaining high subduction velocities to conserve slab integrity. 

Thus, for a strong upper crust, after an initial stage of backward orogenic growth, slab break-off ensues, promoting the rise of hot asthenospheric mantle through the subduction channel and peel-back delamination. In this sense, the orogen grows due to a thermally induced isostatic response of a post-collisional peel-back delamination process (thermally induced orogenic growth).

Finally, we benchmark our models to natural analogues and show that forward orogenic growth models comply well with the width and heights of natural orogenic plateaus.

This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020), and through scholarship UI/BD/154679/2023.