The effect of magma poor and magma rich rifted margins on continental collision dynamics

Rifted margins form when continents rift apart and are commonly characterized by a thinned transition zone between the continental crust and the oceanic crust. This transition zone can display a wide range of characteristics, which primarily depend on the regional tectonic evolution. The velocity and duration of the rifting process as well as the geodynamic setting influence the properties and geometry of the margins, which are often grouped into two main categories: magma-poor and magma-rich.
Magma-rich margins are characterized by large input of mafic melt, while magma-poor margins are characterized by much less magma production during the rifting process, resulting in variations in geometry and rheology of rifted margins worldwide.

Using the finite elements code Citcom, we show how different types of rifted margins can influence the dynamics of continental collision, focusing on the time and depth of slab break-off after collision and the fate of margin material. We compared these models as a function of various parameters (e.g., margin length, density, and viscosity), in order to understand how the architecture of a passive margin affects the dynamics of continental collision.

We find that rifted margins have a noticeable impact on subduction dynamics, as we observe large variability in slab break-off times and depths. In particular, the presence of a rifted margin can delay slab break-off to up to 60 Myr after the onset of collision.
Our results show that a large portion of the weak crust of magma-poor margins is likely to detach from the subducting plate and accrete to the upper plate, while the dense and strong mafic and ultramafic component of magma-rich margins causes most of the margin to subduct and be lost into the mantle, leaving only a small fraction of transitional and oceanic crust at the surface. Therefore, the volume of accreted material is much larger when the margin is magma-poor than magma-rich, which is consistent with geological observations that fossil magma-poor rifted margins are preserved in many mountain ranges, whereas remnants of magma-rich rifted margins are scarce.
Importantly, our results show that rifted margin type controls the architecture of the subsequent collisional phase of the Wilson cycle.