Linking rifted margin crustal shapes with the timing and volume of magma emplacement

The binary magma-rich vs. magma-poor classification of rifted margins was introduced to distinguish between margins showing markedly different crustal architectures, in particular related to the occurrence of magmatic products: the “magma-poor” qualifier is attributed to margins that display a domain of exhumed mantle and whose crustal wedge is exclusively made of continental material, while margins whose continental crust is heavily intruded and overlain by extrusive magmatic flows (e.g., seaward dipping reflections (SDRs) in seismic sections) are regarded as “magma-rich”. Yet, distinguishing between inherited continental crust, newly created magmatic crust and serpentinized mantle in seismic data is challenging due to the comparable geophysical properties (density and seismic velocity). The only interfaces that can usually be identified with some confidence on seismic images are the top of the pre-rift basement and seismic Moho, which allow the determination of the first-order crustal shape of rifted margins. We investigate what the shape of rifted margins can tell us about the timing and volume of magma emplacement during rifting. We use a simple geometric/kinematic model to explore how the volume of magma and the timing of emplacement relative to crustal thinning impact the crustal shape and discuss how this approach may help us to better interpret and understand the tectono-magmatic processes at play during rifting.

We show that crustal shape and inflection points at distal margins can be used to identify magma-poor rifted margins and the occurrence of exhumed mantle. Moreover, the crustal shape and inflection points of magma-poor rifted margins provide direct insights into the dominant processes controlling crustal thinning (e.g., pure-shear stretching, viscoplastic necking, and Coulomb controlled hyperextension) and also the delay of magma emplacement with respect to crustal thinning (e.g., inherited depleted subcontinental mantle, extension rate).

In contrast, shapes of magma-rich margins are more challenging to interpret due to the difficulty to distinguish between continental and magmatic material. We show that different factors may impact the budget and/or timing of magma emplacement and control their distinctive shape, including: (1) the initial conditions from inheritance (e.g., mantle temperature, fertility, and water content); (2) the mode of lithosphere extension (e.g., pure shear vs. depth-dependent lithosphere thinning); and (3) external rift-independent factors (e.g., elevated temperature from mantle plumes).

Crustal shapes allow us to define modes and conditions of crustal thinning at so-called magma-poor rifted margins. In contrast, to interpret crustal shapes of so-called magma-rich rifted margins and understand their tectono-magmatic evolution requires additional information such as timing and budget of magma-emplacement in the crustal wedge, paleo-bathymetry and subsidence history.